A plastic package which houses an electronic component such as a power transistor is typically manufactured by attaching the electronic component to a metal lead frame and then molding a polymer over the assembly to encapsulate the component and lead frame. The electronic component is conventionally attached to the lead frame using either an epoxy or metallization bonding material. Epoxy and metallization bonding materials have a direct affect on the thermal performance of the plastic package. The thermal performance of the package is especially critical for high power applications.
Several factors associated with the bonding material affect package thermal performance. For example, the thickness of the bonding material, commonly referred to as bond line thickness, affects thermal performance. Conventional metallization bonding materials such as PbSn or Pb-free solders are relatively thick, e.g., ranging from about 15 μm to 25 μm or even thicker. Thermal performance of the plastic package decreases as bond line thickness increases. An epoxy can be used to attach an electronic component to a lead frame. However, epoxies have relatively low thermal conductivity of about 20-25 W/m·K, limiting the amount of heat which can be dissipated from the electronic component to the lead frame. Conventional metallization bonding materials such as PbSn or Pb-free solders have a thermal conductivity higher than epoxies of about 30-50 W/m·K. However, an even greater thermal conductivity is desirable for high power applications.
The thermal performance of the plastic package is also affected by the amount of voids in the bonding material. Conventional epoxy and metallization bonding materials tend to have about 5-15% voids. Package thermal performance decreases as the number of voids in the bonding material increases. Package thermal performance further degrades when the lead frame bows during the bonding process and subsequent cool-down period. The lead frame and electronic component usually have different CTEs (coefficients of thermal expansion). As such, the lead frame and component expand and contract at different rates during the bonding process. Conventional lead frames are typically made from a type of conductive material that becomes pliable at bonding temperatures. Accordingly, the lead frame can bow during the bonding process and subsequent cool-down period. An uneven thermal interface between the lead frame and a heat sink attached to the lead frame further degrades thermal performance of the package.